This invention relates to a method and apparatus for electromagnetically processing substances by crushing, mixing and stirring the substances and more particularly to an apparatus equipped with a container for receiving ferromagnetic or non-magnetic conductive working pieces together with substances to be processed by causing a shifting magnetic field to act on and generate a strong random motion of the working pieces.
The background of the invention will be discussed with reference to a prior art device for electromagnetically processing substances of the type described, for example in the copending Watanabe et al application Ser. No. 417,204 filed Sept. 13, 1982 and assigned to the assignee of the present application. As shown in FIGS. 1 and 2, a prior art apparatus comprises a container 1 for containing a number of ferromagnetic or non-magnetic working pieces 2 with substances to be processed. The container faces and is sandwiched between upper and lower shifting magnetic field generators 3, 4. Each of the shifting magnetic field generators 3, 4 is supplied with a coil 5 constructed of three-phase a.c. winding, for example, and generates oppositely directed shifting magnetic fields as shown by arrows .phi.1, .phi.2.
In the apparatus thus constructed, voltage is induced in the working pieces placed in the shifting magnetic fields, allowing an eddy current to flow. Due to reciprocal action between the current and the shifting magnetic fields, an electromagnetic driving force acts on the working pieces in the direction of the shifting magnetic field. In the case where the working pieces are ferromagnetic, a sucking force, in addition to the aforementioned driving force, works in the directions of the shifting magnetic field generators 3, 4. In contrast a repulsive force operates on non-magnetic material. As a result, because of the abovementioned electromagnetic action, the working pieces generate a strong, high speed random motion in the container and act on the substances in the container to simultaneously crush, mix and stir the substances. The same electromagnetic effect may be obtained by installing a shifting magnetic field generator on either the upper or lower side. Such an apparatus as described has various applications including pulverlizing and mixing pulverulent bodies, mixing and stirring liquids, preparing emulsion, forming minute air bubbles, promoting chemical reactions and so on.
The random motion of the working pieces varies with the size and quality of the working pieces, the volume of the container, the percentage fullness of the container with working pieces, the intensity of the shifting magnetic field and so on. According to the results of various tests that have been made, a minimum magnetic field intensity level is required to generate a random motion of working pieces when the volume of a container, the size of working pieces and the percentage fullness of the container with working pieces are fixed. However, operating at this minimum magnetic field intensity level makes it difficult to maintain a stable random motion of the working pieces. Therefore, in order to maintain a practical operating level, a magnetic field intensity sufficiently greater than the aforementioned minimum magnetic field intensity level must be provided. In other words, although the working pieces generate a random motion initially when the apparatus is operated at the minimum magnetic intensity required to generate the random motion, the random movement of the working pieces is gradually reduced because of sucking force and repulsive force produced between the working pieces and the shifting magnetic field generators, and due to magnetic sucking force, collision, frictional force and the like between the working pieces. Ultimately, the working pieces completely cease to move and are put to one side, overlapping one another.
As shown in FIG. 3, most of the working pieces 2, which are ferromagnetic substances, for instance, are attached onto the side of the container 1 without moving as they are being sucked by the shifting magnetic field generators 3, 4. The working pieces 2 line up in a posture like piled up building blocks stuck together under the influence of their magnetic sucking force. Since the working pieces are put in a standstill state, processing of the substances is suspended. This kind of phenomenon may occur even for non-magnetic working pieces. However, if the magnetic field intensity of the shifting magnetic field is increased to a certain value, the electromagnetic driving force in the direction of the shifting magnetic field will increase so that the lined up working pieces will crumple and start their random motion again, to restore to a regular operating condition. As long as a relatively high magnetic field intensity is maintained from the beginning of the operation, suspension of a random motion of the working pieces will not occur.
Accordingly, for practical operation of such an apparatus the magnetic field intensity is sufficiently greater than the minimum magnetic field intensity required to generate a random motion. Continuous operation is obtained by using a high magnetic field intensity so that the random motion is not suspended midway through the operation. However, the conventional method of operation requires the application of excessive magnetic field intensity even during regular operating conditions in which the working pieces would generate sufficient random motion with low magnetic field intensity, which results in excessive power consumption to that extent. More specifically, the magnetic field intensity is proportional to the coil current of a shifting magnetic field generator, and loss in the coil is proportional to the square of the current. According to test results, the magnetic field intensity required to restart the random motion of working pieces suspended during the regular operation while the magnetic field is being applied is 50-80% greater than the minimum magnetic field intensity required to generate the aforementioned random motion. For this reason, the conventional method of operation has disadvantages in low operating efficiency, since approximately twice as much power consumption is required for the regular operation during the whole period of operation, and because of increasing heat loss, a greater cooling capability of a coil cooler is required.